The present invention relates to an imaging device such as an image reading apparatus, a copying machine and a facsimile machine, wherein image information of an original subject is read as electric signals.
There are known image reading apparatus, sometimes referred to as image scanners, for reading character and image information on a document as electric signals. Some image reading apparatus have a document support in the form of a transparent plate of glass, or the like, for supporting a document thereon. The reflected optical image of a document placed on the document support is focused on a photosensitive surface of an image sensor by a lens, and the image sensor converts the image information into electric signals. The document support can be covered with a swingable cover positioned thereabove. The cover is associated with a document holder positioned therebeneath for holding the document down against the document support.
The image sensor includes a linear sensor composed of an array of imaging elements known as pixels. The linear sensor is fixedly positioned below the document support. The image reading apparatus also has an optical system for scanning the document on the document support in a direction normal to the array of pixels, and applying the optical document image to the linear sensor. The linear sensor scans the document in one direction (main scanning direction), while the document image focused on the linear sensor is moved in a direction (auxiliary scanning direction) normal to the array of pixels of the linear sensor.
The optical system includes a scanning unit movable perpendicularly to the array of pixels of the linear sensor. When the scanning unit moves, the document image focused on the linear sensor is moved in the auxiliary scanning direction. Therefore, the linear sensor and the scanning unit jointly scan the document two-dimensionally.
A mirror unit interlinked with the scanning unit is disposed in an optical path between the scanning unit and the lens for keeping constant the distance (optical path length) between the scanned position on the document and the lens irrespective of movement of the scanning unit.
The mirror unit comprises two mirrors lying perpendicularly to each other and having reflecting surfaces facing each other. The two mirrors are positioned to direct the document image, which has been reflected horizontally by a mirror fixed to the scanning unit, backwards to the lens. The mirror unit is movable in ganged relationship to the scanning unit such that the mirror unit moves in the same direction as the scanning unit for a distance that is half the distance traversed by the scanning unit. When the scanning unit moves to scan the document, any change in the length of the optical path followed by the document image is canceled out by the movement of the mirror unit. Therefore, the optical path followed by the document image, which optical path extends from the scanning position on the document to the lens, is rendered constant at all times irrespective of movement of the scanning unit. As a result, the focus of the document image on the linear sensor remains unchanged, and the lens and the linear sensor may be fixedly positioned with respect to each other.
The image reading apparatus of the type described above is used for reading reflective documents, i.e., documents which reflect light, and not suitable for reading transmissive documents, i.e., documents which pass light therethrough. To be able to read transmissive documents, the above image reading apparatus for reading reflective documents needs certain modifications. For example, an image reading apparatus for reading transmissive documents requires a light source positioned above the document support and movable in synchronism with the scanning unit for applying light to a transmissive document. Alternatively, an optical system disposed in the cover for guiding light emitted from a light source below the document support and transmitted through a transmissive document to the linear sensor disposed beneath the document support. To convert the image reading apparatus for reading reflective documents into an image reading apparatus for reading transmissive documents, the cover is replaced with a cover housing an optical system, and the optical system below the document support is modified. Furthermore, the document holder is removed, and the document is sandwiched between two sheets of glass, for example, when placed on the document support.
However, the above modifications are quite complex and time-consuming. Once the image reading apparatus has been modified, it can no longer be used to read reflective documents, and hence lose compatibility between reflective and transmissive documents. Another problem with the modified image reading apparatus is that it is not easy to place the document on the document support, because the document has to be sandwiched between the two sheets of glass.
Generally, the scanning unit is moved over a stroke by an endless belt or wire connected to the scanning unit, and trained around drive and driven pulleys. The endless belt or wire has straight section corresponding to the stroke. When a motor connected to the drive pulley is energized, the endless belt or wire is driven to move the scanning unit.
Since the accuracy, with which a document is read, is governed by the accuracy, of scanning movement of the scanning unit, it is necessary to control the scanning movement of the scanning unit with high accuracy. The ganged structure of the mirror unit and the scanning unit also needs high accuracy. If the drive pulley for moving the scanning unit through the endless belt or wire has an eccentricity error, then the scanning speed of the scanning unit is periodically varied even if the rotation of the drive pulley is precisely controlled.
The mirror unit is ganged with the scanning unit by a pulley rotatably mounted on the mirror unit for rotation about an axis normal to the direction of movement of the mirror unit. A wire or belt is trained around the pulley, with the wire or belt having one end connected to the scanning unit and the other end to an apparatus frame. The mirror unit is normally urged away from the wire or belt by a spring. The spring pulls the mirror unit, placing the wire or belt under tension. With the above ganged arrangement, when the scanning unit moves, the mirror unit moves over a distance which is half the distance traversed by the scanning unit.
As the mirror unit moves upon movement of the scanning unit, the elastic deformation of the spring varies, and so does the biasing force of the spring. The varying bias of the spring results in a change in the resistance to the forces which drive the scanning unit, affecting the scanning movement of the scanning unit in a manner to lower the accuracy with which the document is read. For this reason, it is desirable that the spring has a small spring constant and produces a bias that changes little when the mirror unit moves. The spring with a small spring, constant, is however large in size, and is not preferable from the standpoint of efforts to reduce the size of the entire image reading apparatus.
When the image reading apparatus itself is moved for shipment or the like, the movable components, including the scanning unit and the mirror unit, are usually fastened in position against movement to avoid damage or malfunctioning which would otherwise tend to occur if the movable components were loose and moved by vibrations or shocks. The movable components are generally fastened by screws that are inserted, from outside of an apparatus housing, through holes defined in the apparatus housing into threaded holes defined in the movable components.
Since the movable components are spaced from the inner surface of the housing, it is difficult to thread the screws into the threaded holes of the movable components.
Usually, the lengths of the threaded holes in the movable components are limited. Consequently, the screws used to fasten the movable components have different lengths depending on the distance between the outer surface of the apparatus housing and the movable components. Selective use of the screws of different length is cumbersome, and the screws should be kept in storage in case they are needed when the image reading apparatus is to be moved again.
In the image reading apparatus, the illuminance of the light source or lamp for irradiating the document suffers variations or fluctuations in the main scanning direction, resulting in variations in a bright signal from the image sensor. In addition, even when no document image is applied to the image sensor, the image sensor produces a certain output signal due to a dark current thereof and stray light in the optical system. Such an output signals from the image sensor, aggravated by different sensitivities of the sensor pixels, also gives rise to signal variations or fluctuations in the main scanning direction, resulting in variations in a dark signal from the image sensor. Such signals variations or fluctuations prevent the image sensor from producing correct image signals. Therefore, it has been customary to carry out a corrective procedure prior to a scanning process.
According to the corrective procedure, the image sensor detects image information from a white reference plate to produce an output signals as a bright signal. When the lamp is de-energized, the image sensor produces an output signal as a dark signals. The difference between the levels of the bright and dark signals is normalized.
However, when the lamp is de-energized for the image sensor to produce a dark signals, it takes a certain period of time until the lamp reaches a stable de-energized condition. Inasmuch as such a corrective procedure prior to a scanning process is time-consuming, therefore, the operation of the image reading apparatus to read the document cannot be speeded up.
Color image reading apparatus, for reading a colored document by way of color separation, mostly employs a halogen lamp for irradiating the document as the halogen lamp has linearly varying spectral output characteristics over the visible spectrum range. However, the halogen lamp emits an intensive unwanted thermal radiation, i.e., far-infrared radiation, which heats the document and the image sensor, thus causing a change in the photosensitive characteristics of the image sensor.
Some recent color image reading apparatus have a color sensor comprising an assembly of parallel linear sensors and color filters for passing light of different wavelengths. The color filters are disposed over the photosensitive surfaces of the linear sensors to provide predetermined spectral sensitivities. When a colored document is scanned in one cycle, the light from the colored document is separated into different wavelengths, e.g., those of three colors of red, green, and blue.
To read a colored document with such a color sensor for producing color image information of high accuracy, i.e., good color balance, it is necessary that the color sensor reads the document with uniform sensitivity over the respective wavelength ranges. Specifically, the document should be illuminated with light containing uniform wavelength components and the linear sensor should have uniform sensitivity over the respective wavelength ranges.
Since the spectral sensitivities of the color sensor depend on the spectral characteristics of the color filters used, difficulty is experienced in uniformizing the sensitivities in the respective wavelength ranges. The halogen lamp for irradiating the document actually has different spectral output characteristics in the respective wavelength ranges. Consequently, output signals from the color sensor must be corrected in some way to produce uniform image signals in the respective wavelength ranges. One corrective process would be to vary the amplification degrees for the output signals from the color sensor, but color image information would not be produced with high accuracy according to such a corrective process.
Some electrophotographic copying machines for producing hard copies also have a document support in the form of a sheet of glass for supporting a document to be copied and a swingable pivoted cover for selectively covering the document support. It is necessary that the cover be openable and closable smoothly with small manual forces, and also be closable without disturbing the document which has been placed on the document support. The cover is normally urged into the open position by a spring or the like.
The moment applied to the cover due to its own weight, in a direction to close the cover, varies depending on the angle at which the cover lies with respect to the document support. Accordingly, the spring or the like, which biases the cover to the open position, should have its bias variable as the applied moment varies, so that the spring bias and the moment are held in equilibrium to keep the cover at rest in any desired angular position. Even if the spring bias is selected to counterbalance the moment, however, since a kinetic energy is applied to the cover while it is being opened or closed, the cover may not be held at rest in any desired angular position, and may be closed abruptly especially when the user attempts to close the cover. If the spring bias is increased to prevent the cover from being closed abruptly, then the cover tends to move to the open position. It has been highly difficult to adjust the spring bias to a proper setting.